FIGS. 1A and 1B show top and side views, respectively, of a prior art reactor, configured as a cylindrical reformer 100. The cylindrical reformer 100 includes a cylindrical compartment 101 forming a reaction vessel. The reformer 100 comprises one or more pulse heaters 102A, 102B, each of which comprises a pulse combustor 104A, 104B connected to a respective resonance tube 106A, 106B. As seen in FIG. 1A, the pulse heaters 102A, 102B extend in one direction across the diameter of the cylinder. Air and fuel products enter the pulse combustors 104A, 104B and the combustion products or flue gas exit the resonance tubes 106A, 106B.
The pulse heaters 102A, 102B are of the sort disclosed in U.S. Pat. No. 5,059,404, whose contents are incorporated by reference to the extent necessary to understand the present invention. Such pulse heaters are configured to indirectly heat fluids and solids introduced into a reformer reaction vessel 101. The resonance tubes 106A, 106B associated with the pulse heaters 102A, 102B serve as heating conduits for indirectly heating contents of the compartment 101.
As seen in FIGS. 1A and 1B, a second pair of pulse heaters 108A, 108B are directed at right angles to the first pair of pulse heaters 102A, 102B across the diameter of the compartment. As seen in FIG. 1B, this leaves vertically extending quadrants 136 within the compartment 101 in regions defined by the crossing pulse heaters.
The pulse heaters are immersed in a dense fluid bed 110, which extends from the compartment bottom 112 to approximately the top bed line 114. The bottommost pulse heater 102B is located at a height H1 meters above the distributor 122 to avoid painting the resonance tubes 104B with liquor 118. In some prior art systems, the height H1 is about 2-3 meters.
Spent liquor 118 is injected into the side of the compartment 101 near the bottom of the dense fluid bed 110. Generally speaking, the spent liquor is introduced into the compartment via a plurality of inlets 103 that are circumferentially arranged around the cylindrical compartment 101. Though in FIG. 1B, only four such inlets 103 are shown, it is understood that other numbers of circumferentially arranged inlets may be provided. In other prior art embodiments, the spent liquor may be introduced through the bottom of the compartment 101 through a plurality of inlets more or less evenly distributed across the bottom, perhaps arranged in an array or other pattern.
Superheated steam 120, or other fluidization medium, enters from the bottom of the compartment 101 and passes through a distributor 122. The distributor 122 helps uniformly spread the entering steam 120, which then percolates through the dense fluid bed 110. Product gas 124 leaves from a freeboard area 126 at the top of the compartment 101 after passing through one or more internal cyclones (not shown) used to help drop out entrained bed solids.
FIGS. 2A and 2B show an alternative prior art configuration in the form of a rectangular reformer 200. The rectangular reformer 200 has a compartment 201 with a rectangular cross-section as seen from above (See FIG. 2B). A plurality of pulse heaters 102 arranged in one or more rows pass through this compartment 201. The rows are staggered relative to each other to enhance heat transfer. Each of these pulse heaters 102 comprises a heating conduit in the form of a resonance tube for indirectly heating the contents of the compartment 201.
A distributor 222 is provided at the bottom of the compartment 201, much like in the cylindrical reformer 100. The bottommost pulse heaters 202 are located at a height H2 above the distributor 222. In some prior art systems, this height H2 is again about 2-3 meters. Moreover, just as in the case with the cylindrical reformer, spent liquor 218 is introduced into the side of the compartment 201 near its bottom. Generally speaking, the spent liquor is introduced into the compartment via a plurality of inlets 203 that are arranged along the walls around the rectangular compartment 201. In other prior art embodiments, the spent liquor may be introduced through the bottom of the compartment 201 through a plurality of inlets more or less evenly distributed across the bottom, perhaps arranged in an array or other pattern. Meanwhile, product gas 224 leaves from a freeboard area 226 at the top of the compartment 201. It is understood that the operation of the rectangular reformer 200 is similar to that of the cylindrical reformer 100 described above, in most material respects.
The above arrangements appear to work satisfactorily in small process development scale units. However, they can encounter certain limitations when they are scaled up to larger units.
One issue is the presence of open quadrants (see FIG. 1B) where there are no tubes, or there is free space between pulse heaters bundles (See FIG. 2B), both of which may encourage steam/gas channeling and steam/gas bypassing, thus impairing gas-solid contact and solid circulation rate. Furthermore, the presence of such large vertical channels promotes the formation of large gas bubbles, which, by virtue of rapid increase in size and speed, may damage pipes, tubes, connections and other fixtures within the reformer.
Another issue is that a reduced solids circulation rate leads to longer particle contact times on the resonance tube surface. This hampers particle convection and, in turn, the heat transfer from the tubes. Consequently, the tubes tend to run hotter and this adversely affects the rate at which heat is dumped into the bed and increases the combustion flue gas exit temperature from the pulse heaters 102A, 102B, 202. Additionally, there is a greater propensity for local hot spots, which may lead to smelt formation and/or particle agglomeration and fouling or buildup around a few or many tubes.
Yet another limitation may be that the close coupling of the combustion chamber with the resonance tubes makes it necessary to minimize tube-to-tube spacing, or pitch, and, in turn, the gap between the resonance tubes. This is done to facilitate a reasonable aspect ratio (length to diameter) for the combustion chamber. And since the pulse heater is typically designed as a Helmholtz resonator, it must preserve certain geometric proportions (resonance tube length, resonance tube volume and combustion chamber volume). Experimental data and models for fluidized bed-tube heat transfer indicate significant improvement in heat transfer coefficient with an increase in the pitch or gap between tubes in a tube bundle. This is due to reduced resistance for solids movement with increasing space between tubes, which promotes more frequent surface renewal or particle convection and, in turn, greater heat transfer coefficient. However, the arrangements seen in FIG. 1B limits the gap between tubes to a value that is much lower than optimum from fluid bed heat transfer and operability standpoints.
Furthermore, a majority of combustion and heat release occurs in the pulse combustion chamber. However, combustion continues in the resonance tube albeit at a lower rate due to the lower temperature of the gases in the resonance tube. Residual combustion and heat release in the resonance tube is desirable from a heat transfer standpoint, but unsatisfactory if the combustion is incomplete and gives rise to significant concentrations of CO and unburned hydrocarbons in the exhaust flue gas. The probability of this outcome increases as the fuel-firing rate in the combustor decreases from the design-firing rate.
In addition, upon injection into the fluid bed, the carbonaceous feedstock undergoes drying, devolatilization, char formation and char conversion. In a steam reforming environment, all of these processes are endothermic i.e. require heat input. The greater the bed solids circulation rate and the more uniform the distribution of the feedstock across the bed, the faster the heating rate and the higher the final temperature of the feedstock. This enhances thermal decomposition of organic matter, leads to higher volatile yield and lower tar formation, and char yield. With a fluid bed-pulse heater configuration that encourages gas/steam channeling and bypassing, the solids circulation rate is hampered. This impedes heat transport to the feedstock injection zone, depresses temperature in this zone and promotes tar and char formation.
Yet another issue is that drying, devolatilization, char formation and char conversion processes all compete for heat transfer and mass transfer in the region that is above the distributor but below the bottom pulse heater. All these processes are heat sinks and the entering fluidization medium may be another heat sink if it is steam and is at a temperature below that of the fluid bed. The only heat sources are the pulse heaters and these are significantly removed from the heat sinks by the aforementioned distances H1 and H2 in the prior art embodiments described above. The only link is the solids circulation rate and if this is not up to par, the feedstock injection region starves for heat and the reactor performance suffers.
In addition, both heat transfer and mass transfer are important for satisfactory char conversion. The higher the char temperature and the reactant or steam concentration, the greater the char conversion rate. The region just above the distributor is characterized by high steam or reactant concentration, which is favorable for char conversion, provided the char temperature could be maintained at the fluid bed temperature. Due to feedstock injection and reduced solids circulation rate, the heat supply is limited which is likely to depress the char temperature and in turn the char conversion rate. In the region of the pulse heaters, the heat transfer is good but the mass transfer may be unsatisfactory if the reactant (steam) bypasses due to channeling, again impairing char conversion.
Commercial units generally require deep or tall dense fluidized beds to accommodate the large number of heat transfer tubes. Operating these units in bubbling fluidization regime is rather limiting from heat and mass transfer and gas/solid contact standpoints due to the relatively large bubbles, increased bubble coalescence and the propensity for steam/gas bypassing. Conversely, operation in the turbulent fluidization regime affords good gas/solid contact and excellent heat and mass transfer characteristics. This, however, requires a significantly higher superficial fluidization velocity than that for the bubbling regime. One feasible approach is to select a different heat exchanger configuration and a smaller bed material mean particle size.
In summary, the above-described prior-art configurations afford modularity and are beneficial for certain classes of size or feedstock capacity. However this approach becomes unwieldy for large scale or high feedstock throughput units due to the large number of pulse heaters required, the complexity of interconnections, piping, ducting, etc. and the cost. All of this constrains scale-up.